I think it is interesting that the property can be accessed using reflection. Using the following example I can access the property:
$dt = new DateTime();
$o = new ReflectionObject($dt);
$p = $o->getProperty('date');
$date = $p->getValue($dt)); // returns the value of DateTime::$date

[2013-07-09 11:29 UTC] hanskrentel at yahoo dot de

I wonder how this qualifies as a bug as any object can have variable properties
and accessing undefined ones give notices.
Just sounds like standard PHP behavior here.

[2013-07-09 11:35 UTC] weirdan at gmail dot com

I'd say the bug is not that the property is not present before var_dump'ing
object, but that the property is suddenly created. Unexpected side-effect.

[2014-12-08 17:48 UTC] phpbug at sirdiego dot de

Okay, so I don't know if this is new or relevant in any case, but I found something similar to var_dump.
php > $object = new \DateTime();
php > echo $object->date . PHP_EOL;
PHP Notice: Undefined property: DateTime::$date in php shell code on line 1
PHP Stack trace:
PHP 1. {main}() php shell code:0
php > get_object_vars($object);
php > echo $object->date . PHP_EOL;
2014-12-08 18:44:15
For me this also an unexpected side-effect and would say it is a bug.